This invention relates to a security device having at least one crosslinkable plastic layer and one specularly reflecting metal layer. The invention further relates to a foil material and to a method for producing said foil material.
Optically variable devices, such as holograms, diffraction gratings, etc., have been used for some time for protection against forgery or copy protection by reason of their optical properties varying with the viewing angle. For mass production of such devices it is usual to produce so-called xe2x80x9cmastersxe2x80x9d which have the particular phase information in the form of a three-dimensional relief structure. Starting out from this master one produces by duplication so-called xe2x80x9cembossing diesxe2x80x9d which are used for embossing a long run of the required diffraction structures. Such embossed holograms are usually prepared on a separate carrier as multilayer devices and transferred by means of an adhesive layer to the final object to be protected against falsification, such as a document, passport, credit card, CD, etc. The carrier layer can be removed from the layer structure of the hologram after the latter has been glued to the object to be protected.
The multilayer device applied to the carrier material can be produced e.g. by the method known from U.S. Pat. No. 4,758,296. A sheetlike embossing die circulating on rollers is provided with liquid resin and brought in contact with a plastic carrier material. Simultaneously the liquid resin is cured by means of UV or electron radiation. In a further step the relief structure is provided with a thin metal layer so that the hologram can be observed in reflected light. For transfer to a security document the layer structure is finally provided with a hot-melt adhesive layer which is activated under the action of heat and pressure.
However it has turned out that with the use of embossing lacquers crosslinkable by UV rays the metal layer does not adhere optimally to the embossing lacquer layer. In view of the high requirements placed on the durability of security devices, an improvement of the metal adhesion is desirable not only for UV-cured embossing lacquers but in general also for other lacquers.
The invention is therefore based on the problem of proposing a security device or foil material, and a method for producing it, which ensures improvement of the adhesion of the metal layer to plastic layers.
This problem is solved by the independent claims. Developments are the object of the subclaims.
The essence of the invention is that one or more inorganic auxiliary layers are disposed between the plastic layer and the metal layer. An auxiliary layer here is a thin layer of an element or a compound of elements from main groups II, III, and IV and subgroups 4 to 6 of the periodic system. In particular one can use, as elements, the metals Ti and Cr and, as compounds, oxides of the elements Al, Ti, Zr, Sn, Be, preferably Al2O3, TiO2 or Cr2O3. However one can also use nitrides, borides or carbides, such as TiN, WC. Pure metal alloys, such as NiCr, are also suitable as auxiliary layers for improving metal adhesion on plastics.
It is important in the inventive use that the auxiliary layers lie between the viewer and the reflecting layer of the reflecting security devices. The auxiliary layers are made so thin according to the invention that they do not hinder the reflection of the reflecting layer, i.e. the optical density of the auxiliary layer is in the order of magnitude of 1 or less. With metals such as chromium, or slightly transparent compounds such as TiN, this means that the layer is thinner than 10 nm, preferably in the order of magnitude of 0.5 nm to 5 nm. With very transparent materials, such as Al2O3 or TiO2, the layer must in any case be so thin that the hologram embossing is not filled up, i.e. also generally less than 10 nm. The auxiliary layer can be vapor-deposited or sputtered or applied by other methods, such as PVD (physical vapor deposition) or CVD (chemical vapor deposition) or by photo-CVD, reactive and plasma-enhanced coating methods.
As a reflecting layer one preferably uses aluminum because it is the only metal that can be vapor-deposited cheaply with high reflection and low price. Other metals showing high reflection, such as gold or silver, are expensive; other cheap metals have lower reflection than aluminum but can of course likewise be used depending on the intended use of the security device.
According to a preferred embodiment, the layer structure of the inventive security device is prepared on a carrier layer and the security device then transferred to the object to be protected in the desired form by the transfer method. A plastic foil, such as polyester, is coated in a continuous process with a lacquer layer crosslinkable by UV radiation. An optically variable structure, e.g. diffraction structures in the form of a relief structure, is transferred into this lacquer layer with the aid of an embossing die. The diffraction structures can constitute for example genuine holograms or grating structures, such as cinegrams, pixelgrams, etc.
During the embossing process the lacquer layer is crosslinked by the action of UV radiation. As an auxiliary layer one finally vapor-deposits or sputters on a thin chromium layer which has very good adhesion to the crosslinkable lacquer. To avoid impairing the sharpness of the embossing and thus the brilliance of the optically variable security device, the chromium layer is applied with an optical density of only 0.05, which corresponds roughly to a layer thickness of 1 nm. Over this layer one vapor-deposits an aluminum layer of high reflectivity with an optical density of 2. The adhesive layer necessary for transfer to the object to be protected can likewise be applied to the foil material. It can cover the aluminum layer all over or only in part.
The invention is not restricted to the use of UV-curable lacquers, however. One can use any other embossing layers, such as lacquers initiated by UV light or cured by blue light. The same applies to the adhesive layer. For example one can use hot-melt adhesives or likewise crosslinkable plastic layers.
If adhesion problems likewise occur between the metal layer and the adhesive layer, one can also dispose an auxiliary layer between these layers according to the invention.
According to a further preferred embodiment one uses a transparent auxiliary layer of aluminum oxide instead of the reflecting chromium layer. This layer is deposited on the plastic layer for example by reactive sputtering. Aluminum is sputtered in a very thin oxygen atmosphere of for example 0.02 mb. The layer thickness of the aluminum oxide layer is subsequently a few nm.
The metal layer can additionally be executed in the form of characters or patterns or have gaps in the form of characters or patterns. The partial metalization is usually produced during production of the foil material, e.g. by applying the metalization on the auxiliary layer only in partial areas, e.g. with the aid of masks. Other methods provide all-over metalization which is subsequently removed in the unwanted areas. The auxiliary layer can likewise be removed as well so that it exerts no influence on the optical impression of the device. The same applies when the auxiliary layer is not removed but is transparent and has a refractive index similar to the embossed layer. If the auxiliary layer has high refractive power or an inherent color, however, it can also be used selectively for the optical design of the device.
Using the thus produced foil material one can finally apply security devices with any contours to objects to be protected, such as bank notes, ID cards, as well as other products to be protected against falsification, such as CDs, books, etc.
As mentioned above, however, the invention can be used not only in the case of optically variable security devices but wherever metals adhere poorly to lacquer layers. Thus, one can also perform metal vapor deposition on other security elements such as security threads for bank notes by the above-described method if adhesion problems occur between a metal layer and a directly adjacent plastic layer.